The present invention relates to methods to make integrated circuits including VJFET (vertical junction field effect transistor)-structures and to methods that allow to efficiently integrate a body-diode within the transistor-structure.
A vertical JFET includes a drift-layer, which has to stand the blocking voltage in case the JFET is in its non-conducting off-state. Typically, a JFET includes a steering structure, in which a vertical channel is created to transport the charge carriers when the device is in an on-state (mostly, the channel is an n-channel in between two p-areas located on either side of the channel). When high voltages or currents are to be switched, SiC (silicon carbide) is often used as JFET substrate, as this material has superior properties allowing it to withstand high electric fields without electrical breakdown. Often, frequency converters or voltage changers, which may be implemented using a JFET as current-switching device, require a free wheeling diode to transport the current in a free wheeling state of the JFET, i.e. when the source and drain contacts of the JFET are operated with inverse polarities. In frequency converter or voltage changer applications, it is beneficial to integrate the free wheeling diode into the switching device (e.g., the JFET) itself as a body-diode, such that no external free wheeling diode circuitry is required.
This is particularly of interest, when the diode is robust enough to stand the highest possible currents, such that no external free wheeling circuitry is required at all. This significantly increases the reliability of the system, saves semiconductor area and does furthermore decrease the physical size of the converter resulting in reduced costs. However, to provide for these benefits, the diode has to be placed in the load circuit (i.e. it has to be connected to the source of a JFET) and not in the gate-circuitry. Naturally, a source-contact of a current-switching JFET-structure should have a large area, such as to allow for high currents. Furthermore, the fabrications methods used to provide those JFETs must use technologies or should be designed such as to allow a rather high alignment tolerance, to not harm the electrical performance of the devices or JFETs produced when small, unavoidable misalignment of subsequent production processes does occur. In case such appropriate production or fabrication methods are provided, the production yield can be furthermore increased, resulting in a further decrease of the overall production costs.